There is a class of fuel cells that operate at high elevated temperatures. One type of such fuel cell is a solid oxide fuel cell (SOFC), which comprises two electrodes (anode and cathode) separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at elevated temperatures typically in the order of about 1000° C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO2 and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:H2+O=→H2O+2e−CO+O=→CO2+2e−CH4+4O=→2H2O+CO2+8e−  Anode reaction:O2+4e−→2O=  Cathode reaction:
Known SOFC designs include planar and tubular fuel cells. Tubular fuel cells can be grouped together into a stack to increase output. For example, a tubular stack design published by Siemens Westinghouse Power Generation features tubular fuel cells arranged in a side-by-side rectangular array. The large size of the Siemens Westinghouse fuel cells (typically >5 mm diameter) and the relatively low power density (power output per unit volume) of the stack design makes such a fuel cell stack impractical for small scale applications such as portable electronic devices. Applicant's own PCT application no. PCT/CA01/00634 discloses a method of manufacturing small diameter tubular SOFC that are particularly suitable for small-scale applications. Such fuel cells can be embedded in a solid phase foam matrix to form a stack, as disclosed in Applicant's PCT application no. PCT/CA03/00216.
One of the challenges for SOFC systems is efficient thermal management. It is well known that larger SOFC systems (>5 kW) typically generate more heat than needed to keep the fuel cell stack at a suitable operating temperature, and therefore, need efficient heat removal techniques to prevent overshooting the temperature of the stack. In contrast, smaller SOFC systems generate less heat and consideration must be given in certain situations to retaining enough heat to keep the stack sufficiently warm. When a stack's size is reduced, the stack's ratio of outer surface area to volume tends to increase, which results in an increase in potential heat loss relative to rated power output. As SOFCs have to operate at high elevated temperatures, it is important to keep the stack and incoming reactant gases at suitable elevated operating temperatures. Inadequate thermal management can result in significant amounts of generated heat to be lost, such that heat from an external source must be used to heat the incoming reactant streams as well as to keep the stack within its operating temperature range. Such external heat sources constitute a parasitic load on the SOFC system which reduces the operating efficiency of the system.
A heat exchanger design known as a “Swiss roll” was conceived about thirty years ago by Felix Weinberg of Imperial College London. This heat exchanger had a supply fuel flowing in a channel running parallel with a channel carrying hot exhaust. The channels were rolled into a spiral, which had the effect of substantially increasing the internal surface area that was exchanging heat, as well as minimizing external surfaces that were losing heat. More recently, a team at the California Institute of Technology led by Sossina Haile has experimented with installing a fuel cell within a Swiss roll heat exchanger. Known Swiss roll heat exchangers are typically rigid structures having complex geometries that are fabricated from high-temperature tolerant materials such as titanium and ceramic. The manufacture of such heat exchangers and the integration of the fuel cell within the heat exchanger are laborious and not commercially practical for large scale manufacture.